1. Field of the Invention
The present invention generally relates to the field of vacuum evaporation of metals and more particularly to those cases where this operation is to be done continuously in order to coat a continuous substrate, such as a wire or strip.
2. Description of the Prior Art
Deposition, co-deposition, coating, or protection can all be obtained by processes such as condensation or ionic plating. These techniques are used in the deposition of metals in the vapor phase and apply to many areas such as microelectronics, coating of plastics, protection of metal parts against corrosion or the manufacture of composite materials.
The evaporator sources are generally static, i.e., the metal present at the beginning is the source and is partially or completely evaporated during the coating operation. Thus, it is necessary to periodically recharge the evaporator source, which involves burdensome operations because the the vacuum chamber must be opened in order to introduce a new quantity of metal and then pumped out again to restore the vacuum. This problem can be solved by using high-capacity static sources which contain a large quantity of metal to be evaporated. But, with this solution, high heating power is necessary in order to evaporate that amount of metal, and monitoring the regularity of the evaporation rate as a function of time is difficult. These problems are particularly troublesome in the case of an extremely large quantity of metal, as well as in the case of a long operating period, for example, when the substrate is continuous and unwinds before one or several evaporation sources.
Continuous operation sources have already been used. For example, the metal to be evaporated may be a wire which supplies, as the operation progresses, a crucible heated by the Joule effect, by high frequency, or by any other means. The wire is melted as it arrives in the continuous liquid metal phase in the crucible, and it is the liquid mass which vaporizes. This system is currently used in the area of aeronautics for the deposition of a aluminum used to protect parts against corrosion, as well as in the manufacture of composite materials, for example, aluminum-carbon fibers. Japanese Pat. No. 120,876 describes a process of this type.
Although using wire makes possible the evaporation of large quantities of metal and long term operation without requiring too much heating capacity, this method causes problems when the metal used is magnesium. This metal has a very low boiling point (1107.degree. C. under normal temperature and pressure conditions) and, under the vacuum evaporation conditions generally used in vapor phase deposit techniques, the liquid phase does not occur between the solid phase and the vapor phase. That means that the supply wire may not soak in a bath of melted metal, but is immediately vaporized once it comes into contact with the crucible or once its temperature is sufficiently high. The hot surface from which the vapor emission is produced is thus significantly reduced in relation to the one that can be obtained with the use of a liquid mass; the result is that the deposition speed is much slower.
Another factor to be taken into consideration is that magnesium wire is not a common product and that it would have to be custom manufactured. This would cause a significant increase in the cost of the finished product. This would be particularly unfortunate because the substrate (spread-out fibers made of carbon, silicon carbide or aluminum oxide in the case of composite materials, for example), which is, in fact, the main part of the finished product, would be less expensive than the matrix. Finally, the low mechanical resistance and the lack of rigidity of magnesium wire would significantly complicate the supply system.
These problems led to the origin of the idea to supply the evaporator sources with metals in the form of powder, granules or any other divided form permitting quasi-continuous supply. However, in the case of magnesium, the high vapor tension of this metal in the usual evaporation conditions of vapor phase deposition processes, as well as its low density (1.74 g/cc), cause upheaval and significant movements in the powder grains as soon as they are introduced into the heated crucible. These movements are most likely due to the abrupt formation of vapor when the grains are heated rapidly, because their heat generating capacity is 0.25 cal/g/.degree.C. This abrupt formation of vapor transmits a significant amount of movement to the powder grains, and thess movements can be maintained by convection. Thus, the powder grains are rapidly ejected from the crucible, resulting in poor yield, yield being defined as the ratio between the quantity of metal evaporated and the quantity of metal introduced. This yield may be as low as approximately 1 to 2 percent. In addition, the grains ejected in this way may be projected onto the supply system and clog it in the relatively long term through coalescence of the ejected grains as well as condensation of metal vapor, which causes interruptions in production.